Our early human ancestors began to walk on two legs a couple of million years ago, abandoning their old life of swinging between tree branches and perching in the leafy forest canopies of Africa. Now, a new study suggests that this transition from the trees to the ground was a long one. It appears that our ancestor A. Afarensis — typified by the 3.2 million-year-old "Lucy" fossil skeleton — lived at least part of her life in the trees. But she also walked on two legs.

What this means is that the whole idea of a simple "missing link" between humans and our ape cousins is false. There was no one, single moment when humans leapt from the trees to find a new existence on land. It happened gradually, over millennia, with different individuals from different species testing out what it would mean to live far from the protection of sheltering forests. Instead of thinking of our transition to walking as a "missing link," it would be more accurate to say the transition was a long chain, in which one kind of life shaded into the other very gradually.

We know now that Lucy and other members of her species probably lived at least part time in the trees because of a new discovery about the structure of their shoulders. David J. Green, an anatomist, and Zeresenay Alemseged, an anthropologist, are among a group of scientists who have been able to examine the shoulder joints of Lucy and another A. Afarensis skeleton much more thoroughly than previous researchers — partly because experts have finally been able to separate these bones from the rocky material in which they were found. It turns out Lucy's shoulder sockets were similar to those of modern apes. The opening for the shoulder joint points upward, which means this species probably spent a lot of time reaching and climbing the way apes do today. Modern humans, by contrast have an opening for their shoulder joints that angles downward at birth, and slowly rotates so that there are openings on the sides.

Given how much safer it would have been for the small, slight A. Afarensis in the trees, one has to ask: Why leave at all? Anthropologists believe that everything from changing weather patterns to sheer curiosity would have driven these early humans to seek food and shelter on the ground. And once individuals and groups were acclimated to walking, it made humans the most versatile primate on Earth. We didn't need to rely on tree cover to get around. We could just stand on our own two feet and walk somewhere.

If you've ever wondered why humans conquered the Earth, the answer starts in the long evolutionary chain that includes Lucy and her species. Their exploratory ventures beyond the trees eventually led to a species of early humans called Homo erectus, who walked all the way up the east African coast and straight into the Middle East and Eurasia. Sure, our big brains helped us become a dominant predator — but long before that, we conquered with our unique style of getting around on two legs.

What this new discovery highlights is the degree to which evolutionary changes don't always have an easy beginning and ending. We'd like to believe there was a simple missing link between ape-like humans and human-like humans — perhaps a single species that provides a nice bright line between us and chimps. But the more we learn, the more we realize there is no species like that. There are species who started the exploration process, taking those first treks across the treeless savannahs, and there are species who continued that process.

To truly grasp how evolution works, we need to let go of the myth that there were radical distinctions between early human species. Evolution is a messy process, and it will never stop being messy. Modern humans are continuing the exploration process that A. Afarensis launched millions of years ago — and in another million years, we may have evolved an entirely new way of getting around. If we have, it won't be because one day everybody woke up with a brand-new bone structure. It will be because over thousands of years, and millions of false starts, we slowly and irregularly transformed into a kind of human distinct from the ones who lived before.

You can read the new scientific paper about A. Afarensis remaining part-time in the trees in Science magazine.